Applicant's employer has identified mechanical resonances occurring in what is referred to in the art as a line narrowing module (“LNM”) or line narrowing package (“LNP”). Such an LNM may contain, e.g., a dispersive wavelength selective optical element, e.g., as is contained in single chamber laser systems, e.g., Nanolith 7000 laser systems sold by applicant's employer and assignee Cymer, Inc. and/or two chamber laser systems such as master-oscillator/power amplifier (“MOPA” laser systems such as the XLA-XXX series of MOPA laser systems, e.g., beginning with the XLA-100, introduced in about 2003. Recently applicant's assignee has introduced a version of the multi-chambered laser systems utilizing a power ring amplifier (“PRA”) for the amplifier portion, i.e., a “MOPRA”.
The dispersive wavelength selective optic of choice has been a blazed eschelle grating in a Littrow configuration, with center wavelength being selected by changing the angle of incidence of light emitting from the lasing chamber upon the grating. The grating acts as the fully reflective rear mirror of the lasing cavity of the MO or the single chamber laser system and along with, e.g., the use of an aperture selects the center wavelength of the laser system output and incidentally, depending on such things as the wavefront of the light incident on the grating and in combination with the shape of the grating also selects bandwidth of the laser system output as well.
Recently, for various reasons, applicant's employer has changed from a fully maximum reflective mirror (“RMAX”) to a series of prisms for changing the angle of incidence of the light on the grating, e.g., for use with a longer grating for better center wavelength and bandwidth control. The series of prisms both increases the magnification of the beam of pulses of light incident on the grating and controls the angle of incidence. The final prism in the series of prisms, e.g., four prisms, in the arrangement selected by applicant's assignee, but could be more or less than four, may be selected to be the one for coarse control of the angle of incidence, e.g., by rotating the prism about a point of pivoting of the prism motion. One or more of the other prisms may also be rotated for fine center wavelength control or the arrangement could also be reversed, so that, e.g., the final prism is for coarse control and the other prism(s) for fine control.
To accomplish the rotary motion, applicant's assignee has adopted a flexured mounting for the final prism in the series, as explained in more detail below. To enable the prism to rotate the flexure mounting also makes the prism susceptible to vibration which can be very detrimental to center wavelength control and/or stability and adversely impact bandwidth control as well.
Damping mechanisms exist to lessen the effect of vibration on the prism positioning, which are typically constructed similarly to vibration isolation mountings, i.e., with an elastomeric material sandwiched between rigid mounting plates, such as made of steel or ceramic, except that only one side of the damping mechanism has a mounting plate and the other typically is formed by a mass that may, e.g., be tuned to a desired range of frequencies of vibration desired to be damped.
At least two problems arise from the use of such damping mechanisms in an LNM of the type referred to herein. First, the elastomeric material, e.g., Viton™ is susceptible to deterioration when exposed to DUV light, which is the type of light being produced by the laser system and of which the center wavelength and bandwidth are being controlled in the LNM, resulting in DUV outside the bandwidth selected by the LNM reflecting inside the LNM and adversely impacting the structural integrity of the elastomeric material. In addition to this structural deterioration, and the impact on damping functionality of the elastomer-containing damper, the material is elastomer susceptible to outgasing which can coat or otherwise damage the optical elements, prisms and grating and/or otherwise adversely impact the optical performance of the LNM, and, e.g., shorten LNM lifetime. Deterioration of the ability to maintain center wavelength and/or bandwidth control within the extremely tight tolerances allowed in modern day utilizations of DUV laser light sources, e.g., integrated circuit photolithography processes, either from limited life of the damping mechanism or deterioration of the functioning of the optical elements or both, causing the need to replace the LNM more often than normally required can have a severe impact on the cost of operation of the laser system over its lifetime, which is detrimental to the effectiveness and/or efficiency of use of the laser system over its lifetime.
A technique to suppress resonances commonly applied by Carl Zeiss SMT is the addition of damped oscillators or mass dampers (so-called “Tilgers”), which may, e.g., consist of mass elements which are connected to the vibrating component via damping pieces of an elastomeric material, e.g., Viton/rubber. A mass, created, e.g. by screw nuts on a 2 mm-thick Viton slab, in accordance with a rough estimate for an appropriate geometry involved desired to be damped, and such things as its mass and resonant frequency(ies), may be, e.g., glued to the side(s) of a flexure mounting where one can expect the largest amplitude of the oscillation to occur, e.g., at the portion that is allowed to flex most with the bending of the flexure mounting.
Applicants propose to resolve the inability of using damping mechanisms containing elastomeric materials in the environment and for the functionality of an LNM according to aspects of embodiments of the disclosed subject matter.